The present invention relates to a sintered molding, consisting of partially stabilized zirconium oxide as the sole or principal component mixed with additional ceramic materials, in which magnesium oxide and yttrium oxide are uniformly distributed as stabilizing oxides, the crystal phase composition of the zirconium oxide including the cubic, tetragonal and monoclinic modifications, but the content of the monoclinic modification amounting to no more than 5% by volume of the total phase composition. The invention moreover relates to a method for the production of the sintered molding, and to its use. A sintered molding of partially stabilized zirconium oxide is known from DE-A-23 07 666 owned by the Assignee of the present invention. In addition to the magnesium oxide cited for the achievement of optimum properties, mention was made therein of yttrium oxide, along with the oxides of cerium, lanthanum, ytterbium, and titanium, as well as mixtures of these oxides, but without having in view any specific advantageous action of these additional oxides. For the achievement of an improved strength under high temperatures, this proposal is aimed at very high cubic phase contents ranging from 75 to 95%, without stating the nature of the rest of the phase contents. A flexural breaking strength of 63 kp/mm.sup.2 corresponding to about 618 MPa is given as the maximum strength, but it has been found in the meantime that this very good strength is greatly reduced by extended thermal stress, so that in many cases this ceramic cannot be used. Another disadvantage lies in the complicated method of manufacture which sets out from prestabilized zirconium oxide, unstabilized zirconium oxide and stabilizing metal oxides.
A study of the use of different metal oxides in the stabilization of zirconium oxide is described in the article, "Stabilization of Zirconia with Combined Additives, A Study of the Solid Solution Stability" (published in "Refractories" (1970) pp. 723-726)). A mixture of 92 mol-% of zirconium oxide, 5 mol-% of magnesium oxide and 3 mol-% of yttrium oxide has, according to this study, an exclusively cubic modification, but with a strength of 2000 kg/cm.sup.2 corresponding to 196 MPa. Thus, it is still well below the strength of the zirconium oxide described in DE-A-23 07 666.
In EP-A-13 599 a partially stabilized zirconium oxide is described in which nothing but magnesium oxide is provided as a stabilizing metal oxide, in the amount of 2.8 to 4% by weight. The phase composition of this zirconium oxide is given as 2 to 10 wt.-% tetragonal and 0.5 to 20 wt.-% monoclinic. Tetragonal and monoclinic phases are present as a segregation in grains consisting otherwise of the cubic modification. As it has been found on the basis of research, articles made of this partially stabilized zirconium oxide do not have a sufficient high-temperature stability for many applications.
A number of proposals have become known for improving the heat resistance of the known partially stabilized zirconium oxides. EP-A-36 786 provides for a partially stabilized zirconium oxide containing 2 to 7 mol-% of yttrium oxide as a stabilizing oxide in order to improve the high-temperature stability in the range from 200 to 300.degree. C. Up to 30 mol-% of the yttrium oxide can be replaced by oxides of ytterbium, scandium, niobium, or samarium, or by calcium oxide or magnesium oxide. Also, up to 30 wt.-% of aluminum oxide, silicon oxide and aluminum silicate can also be added as additional ceramic substances. The crystal phase composition is given as cubic/tetragonal, cubic/tetragonal and up to 20 vol.-% monoclinic, tetragonal, tetragonal, and up to 20 vol.-% monoclinic or cubic, so that no precise conclusion can be made as regards the cubic/tetragonal percentages. To obtain the zirconium oxide in the tetragonal modification, provision is made according to EP-A-36 786 for the zirconium oxide to be present in the sintered molding in a grain size not exceeding 2 microns, and preferably the grain size is to be even less than 1 micron. To arrive at such a fine grain size, EP-A-36 787 provides for the use of zirconium oxide and of yttrium oxide starting compounds, i.e., for example, the corresponding chlorides, nitrates or oxalates, which are first mixed, then thermally decomposed, and then ground wet, in order to arrive at an extremely fine starting powder with which the production of a sintered molding of high density and sufficient strength with a grain size less than 2 microns is possible by using a sintering temperature below 1500.degree. C. The important difference between the material described in EP-A-36 786 and the above-discussed partially stabilized zirconium oxides consists in the structure. Due to the extremely small grain size in conjunction with the relatively low sintering temperature, according to EP-A-36 786 a structure is obtained in which the individual grains of the structure are in the cubic or tetragonal, or even monoclinic modification, depending on their size and their content of stabilizing oxides.
This complicated method of production results in an extremely expensive starting material and has heretofore impeded the widespread use of sintered moldings consisting of this material. It has been found, furthermore, that the thermal load-bearing strength is often still unsatisfactory. Another disadvantage of this extremely fine-grained zirconium oxide material consists in its high rate of creep at high temperatures, which is about 1 to 2 powers of ten greater than it is in coarse-grained, partially stabilized zirconium oxide materials.
TZP ceramics are also described in the U.S. Pat. Nos. 4,544,607, 4,565,792 and 4,587,225 and in the Japanese Patent Publications Nos. 58-36976 and 58-32066. These proposals share the disadvantage that the strength of the respective materials falls off precipitously under high temperature conditions because, under such conditions, the crystals, which are present in the tetragonal modification, grow and are converted into the monoclinic modification.
According to another proposal disclosed in DE-A-33 45 659 for the improvement of the known sintered moldings of partially stabilized zirconium oxide, provision is made for coating a sintered molding with a more highly stabilized, thin layer or one that is mostly in the cubic phase. The base bodies can be either the so-called TZP zirconium oxide sintered bodies, as described in EP-A-36 786, or the common zirconium oxide ceramics stabilized with magnesium oxide described in EP-A 13 599. The contemplated improvement, however, relates only to a relatively low temperature range up to 400.degree. C, at which the degradation process of the so-called TZP ceramics, occurring under the influence of moisture, is said to be limited by means of the proposed coating. A disadvantage of this proposal is the slight thickness of 0.1 to 200 microns, preferably 0.3 to 30 microns, of the outer protective coating. For if this coating is removed from parts subject to wear, the known disadvantages are to be expected. Another disadvantage lies in the difficulty of preparing precisely dimensioned parts, because the sintering process is mostly followed by a mechanical working of the sintered molding, e.g., by grinding its surface. An applied protective coating would in most cases be thus removed, even if its thickness were 200 microns. If the coating process described is used for a conventional zirconium oxide ceramic partially stabilized with magnesium oxide (Mg-PSZ), it can be assumed that only very small percentages of monoclinic phase can be measured at the surface, close to the protective coating, even after a relatively long exposure to high-temperature stress, while in the interior of the sintered object, that is, where the outer protective coating is of no influence, the decomposition or transformation to the monoclinic phase described even in the case of the known zirconium oxide ceramics partially stabilized with magnesium oxide will take place, since here a migration of magnesium oxide into the area of the grain boundary takes place.
In WO 83/04247 it has already been proposed to add to a zirconium oxide partially stabilized with 3 to 3.65 wt-% of MgO another 0.05 to 1 wt-% of strontium or barium oxide. In this disclosure both high-strength zirconium oxide ceramics and those which can be used at room temperature are described, as well as those which are usable at high temperature but may have a lower strength. The microstructure of the zirconium oxide ceramic, depending on the temperature range in which the ceramic is to be used, is to be such that the monoclinic zirconium oxide content, present as a segregation in the cubic matrix, will be higher as the temperature range in which the zirconium oxide ceramic is used is increased. For a zirconium oxide ceramic that can be used for brief periods at high temperatures, a monoclinic phase content of 35 to 65 vol-%, measured at the mirror-polished surface, is accordingly required. The monoclinic zirconium oxide content present at the grain boundaries can, in that case, amount to as much as 30 vol-%, but preferably to only 20 vol-%.
Although that disclosure refers to use of the refractory zirconium oxide produced therein at temperatures ranges exceeding 1000.degree. C, no information regarding the flexural strengths found at such temperatures is contained therein.
The disadvantage of this material consists in the poor stability of shape at high temperatures. This can be understood because a high content of monoclinic segregations forms even in the making of the ceramic, and the size of the segregations still present in the tetragonal modification (phase) has already greatly increased. In the event of further thermal stress, on account of the growth of the tetragonal segregations which this entails, segregations at first still present in tetragonal modification are converted to the monoclinic modification, with an increase in volume.
Setting out from this state of the art, the problem to which the present invention is addressed consists in developing a sintered molding consisting of partially stabilized zirconium oxide or containing a partially stabilized zirconium oxide as the predominant component, which will have an excellent flexural strength and resistance to thermal shock even under the influence of high temperature, so that it will be possible to make components from such a zirconium oxide which can be used over a long period of time even at temperatures of more than 1000.degree. C. The invention also aims to make available a zirconium oxide which will have good dimensional or shape stability at high temperatures. The invention also is in a manufacturing process of reasonable cost for the production of a partially stabilized zirconium oxide that has good shape stability.